I found that for the 67+Mark 4 services from Crewe to Shrewsbury by TfW, it's cheaper to get a first class ticket rather than a standard class (as shown below for an adult single booked a week in advance with no railcard - though I'm pretty sure it works no matter when you book, as long as the service offers first class)

Why does this happen? Is it something to do with the government subsidies for that line, or just a weird quirk with the fare structure. And are there any other examples of this (or something similar) happening elsewhere?

Why does this happen? Is it something to do with the government subsidies for that line, or just a weird quirk with the fare structure. And are there any other examples of this (or something similar) happening elsewhere?